The Mark XXLII probably didn't have its own arc reactor yet because it was an incomplete prototype, and it had to be independent of Tony's reactor because of its remote assembly nature.

That's plausible. But it wasn't properly explained in the movie... Just like there was no logical explanation with the way

Spoiler!!! Click to Read!:

Iron Patriot was taken out by one extremis soldier heating up one arm. The overheating clearly didn't incapacitate the suit because minutes after being heated up by Killian it was used to fly to AF1. Was it AIM previously tinkering with the suit AI ? That's a possibility, but the movie did not do a good job of explaining that.

__________________"A shared universe, like any fictional construct, hinges on suspension of disbelief. When continuity is tossed away, it tatters the construct. Undermines it."
-- Peter David